The present invention relates to a control apparatus for an electrical power steering apparatus for applying steering assistance force by means of a motor to the steering system of an automobile or vehicle, and relates more particularly to a control apparatus for an electrical power steering apparatus whereby torque ripple is reduced and steering feel is improved using a DC motor or a brushless motor.
Electric power steering systems that use the torque from a motor to assist the steering system of an automobile or other vehicle use a transfer mechanism such as gears or belts to transfer drive power from the motor via a speed reducer to assist turning the steering shaft or rack shaft. Such conventional electrical power steering systems use motor current feedback control to accurately generate the assistance torque (steering assistance torque). Feedback control adjusts the voltage applied to the motor so that the difference between the current control value and the detected motor current becomes smaller, and adjusting the voltage applied to the motor is generally accomplished by adjusting the duty ratio of PWM (Pulse Width Modulation) control.
To describe the general structure of an electrical power steering system with reference to FIG. 1, a shaft 2 of a steering wheel 1 is connected to tie rod 6 of the steering wheels by way of speed reducer gear 3, universal joints 4a and 4b, and rack and pinion mechanism 5. A torque sensor 10 for detecting the steering torque of steering wheel 1 is disposed to the shaft 2, and a motor 20 for assisting the steering power of the steering wheel 1 is linked to the shaft 2 by intervening a clutch 21 and speed reducer gear 3. Power is supplied by way of ignition key 11 and relay 13 from a battery 14 to a control unit 30, which controls the power steering system; the control unit 30 calculates steering assistance command value I of the assistance command based on steering torque T detected by the torque sensor 10 and speed V detected by a vehicle speed sensor 12, and controls the current supplied to the motor 20 based on the calculated steering assistance command value I. The clutch 21 is turned on and off by the control unit 30, and is on (engaged) during normal operating conditions. When the power steering system is determined by the control unit 30 to have failed, and when the power (voltage Vb) from the battery 14 is turned off by the ignition key 11 and the relay 13, the clutch 21 is off (disengaged).
The control unit 30 consists primarily of a CPU; the general functions executed by a program in the CPU are as shown in FIG. 2. For example, phase compensator 31 does not indicate a phase compensator provided as a separate hardware component, but rather indicates a phase compensation function executed by the CPU. To describe the functions and operation of the control unit 30, steering torque T detected and inputted by the torque sensor 10 is phase-compensated by a phase compensator 31 to improve the stability of the steering system, and the phase-compensated steering torque TA is then inputted to a steering assistance command value calculator 32. The steering assistance command value calculator 32 determines the steering assistance command value I, which is the control target of the current supplied to the motor 20, based on the input steering torque TA and speed V. The steering assistance command value I is inputted to a subtracter 30A and a differential compensator 34 of the feed forward system for improving response speed, the deviation (Ixe2x88x92i) of the subtracter 30A is inputted to a proportional calculator 35, and the proportional output is inputted to an adder 30B and an integral operator 36. Outputs from the differential compensator 34 and the integral operator 36 are also additively inputted to the adder 30B, and the resulting current control value E, that is, the sum obtained by the adder 30B, is inputted to a motor drive circuit 37 as the motor drive signal. Motor current i of the motor 20 is detected by a motor current detecting circuit 38, and the motor current i is inputted to the subtracter 30A and is fed back.
To describe the structure of the motor drive circuit 37 with reference to FIG. 3, the motor drive circuit 37 comprises an FET gate driver 371 for driving the gates of field effect transistors (FET) FET1 to FET4 based on the current control value E from the adder 30B, an H-bridge consisting of FET1 to FET4, and a step-up power supply 372 for driving the high side of FET1 and FET2. FET1 and FET2 are switched on and off by a PWM (pulse width modulation) signal of duty ratio D1 determined based on the current control value E, and the size of current Ir actually flowing to the motor 20 is controlled. FET3 and FET4 are driven by a PWM signal of duty ratio D2, defined by a specific first degree equation (D2=axc2x7D1+b, where a and b are constants) in the range where duty ratio D1 is small, and go on/off according to the direction of the motor 20 rotation, which is determined by the sign of the PWM signal, after duty ratio D2 reaches 100%. For example, when continuity exists through FET3, current flows through FET1, the motor 20, FET3, and resistor R1, and forward current flows to the motor 20. When continuity exists through FET4, current flows through FET2, the motor 20, FET4, and resistor R2, and reverse current flows to the motor 20. Therefore, current control value E from the adder 30B is also PWM-output. The motor current detecting circuit 38 detects the size of the forward current based on the voltage drop at both ends of resistor R1, and detects the size in the reverse direction based on the voltage drop at both ends of resistor R2. The motor current i detected by the motor current detecting circuit 38 is inputted to the subtracter 30A for feedback.
In conjunction with the desire for high output in such conventional electrical power steering systems, there has also been demand for systems achieving a sense of high quality steering, as well as a small size due to layout considerations. To achieve a sense of high quality, it is necessary achieve a feeling of smooth steering. The torque ripple of the motor is generally the factor that determines whether smooth steering is achieved, and measures have been conventionally taken to reduce the torque ripple of the motor as a means of achieving smooth steering. However, conventional technology for reducing motor torque ripple typically reduces the center angle of the magnet in the motor or applies a skew angle to the magnet. In either case, if torque output is maintained, overall motor size increases such that the desire for smaller size cannot be satisfied.
On the other hand, rare earth magnets with high residual magnetic flux density have been used in brushless motors in conjunction with demand for high output and small size in conventional electrical power steering systems. However, smooth steering in response to the operator""s manipulation of the steering wheel is needed even with brushless motors, and there is therefore a great need to reduce the motor""s torque ripple and cogging torque.
A motor with a small skew angle and magnetization in which the magnetic flux density of the magnetization is close to a trapezoidal wave is needed to reduce torque ripple in a square wave drive brushless motor. However, because this need conflicts with reducing motor cogging torque, it is in reality difficult to reduce torque ripple. Furthermore, as a result of a degraded steering feel due to the effect of torque variation resulting from a shift in the commutation position (due to variation in the relative positions of magnetization and the coil, and the effect of position detection precision, as a result of such factors as armature reaction), and the effect of torque variations due to current variation during commutation, practical use of a high torque constant is difficult, and it is even more difficult to satisfy variation, position detection precision, current variation, and other requirements.
Furthermore, in the case of a sine wave drive brushless motor high resolution is required in the rotor position detector in order to reduce torque ripple, and to compensate for the effects of armature reaction in a motor with a high motor torque constant, position detector and CPU cost become high because of the advanced calculation capabilities and compensation data memory requirements.
The present invention was conceived with consideration for the above, and an object of the present invention is to provide a control apparatus for an electrical power steering apparatus comprised to achieve high output and a small size in the motor of the electrical power steering system, alleviate variation in the relative positions of magnetization and the coil, improve the steering feel (reduce the torque ripple feeling), and reduce apparatus cost.
The present invention relates to a control apparatus for an electrical power steering apparatus that controls a motor applying steering assistance force to the steering mechanism based on a current control value calculated from the motor current and a steering assistance command value calculated based on the steering torque produced on the steering shaft, and an object of the present invention is to restrain the torque ripple of the motor (a DC motor or brushless motor) within 10%. Moreover, even greater effectiveness can be achieved when the hysteresis width of the torque detecting means is 0.3 Nxc2x7m or less, the response frequency is 20 Hz or less in cases, and the torque control frequency band of the control apparatus is 20 Hz or greater.
As a result, torque ripple factors in a DC motor or brushless motor of the electrical power steering system are analyzed, and torque ripple is reduced by manipulating means corresponding to these factors, in the present invention. Furthermore in the present invention, torque changes and torque variation in a square wave drive brushless motor are suppressed, and high frequency components that cannot be suppressed are absorbed by the damping effect of the torque transmission system. Low frequency components such as torque pulses are allowed to the level that they can be controlled by the torque control system, torque ripple is detected using a torque sensor with low hysteresis, and the effects thereof are suppressed by the control system. It is therefore possible to achieve a high output, small DC motor or brushless motor, and reduce torque ripple at low cost without sacrificing cogging torque.